1. Field of the Invention
This invention relates to a method for making tactioselective polyolefin homopolymers and copolymers using hafnocenes with selected symmetry conditions.
More particularly, this invention relates to a method for making tactioselective polyolefin homopolymers and copolymers using hafnocenes with selected symmetry conditions in condensed phases at temperature above about 50.degree. C.
2. Brief Description of Related Art
Polymerization of vinyl monomer, both mono-olefins and conjugated dienes, has focused on transition metal catalysts since the work of Ziegler and Natta. These catalysts are based on a central transition metal ion or atom surrounded by a set of coordinating ligands and modified by various co-catalysts. These polymerization systems when brought in contact with addition polymerizable monomers polymerize the monomers into polymers.
By controlling the nature of the ligand system, the central transition metal ion or atom, and the co-catalyst, highly active catalytic agents can be made. In addition, catalysts can be made that yield polymers with high degrees of addition regularity and in the case of non-ethylene type monomers, stereoregular or tactioselective and/or tactiospecific polymers can be made.
Atactic polymers exhibit no regular order of repeat unit orientation in the polymer chain, i.e., the substituents are not regularly ordered relative to a hypothetical plane containing the polymer backbone (the plane is oriented such that the substituents on the pseudo-asymmetric carbon atoms are either above or below the plane). Instead, atactic polymers exhibit a random distribution of substituent orientations.
Besides metallocene catalyst that produce polyethylene and atactic polyolefins, certain metallocenes are also known to produce polymers with varying degrees of stereoregularity or tactiospecificity, such as isotactic, syndiotactic, and hemi-isotactic polymers which have unique and regularly repeating stereochemistries or substituent orientations relative to the plane containing the polymer backbone.
Isotactic polymers are typically described as having the substituents attached to the--asymmetric carbon atoms, oriented on the same side relative to the polymer backbone, i.e., the substituents are all either configured above or below the plane containing the polymer backbone. Isotacticity can be determined through the use of NMR. In Bovey's NMR nomenclature, an isotactic pentad is represented by . . . mmmm . . . with each "m" representing a "meso" dyad or successive monomer units oriented with the substituents oriented on the same side relative to the polymer backbone. As is well known in the art, any deviation, disruption, or inversion about a pseudo-asymmetric carbon in the chain will lower the degree of isotacticity and crystallinity of the polymer.
In contrast, the syndiotactic structure is typically described as having the substituents, that are attached to the pseudo-asymmetric carbon atoms, pseudo-enantiomorphically disposed, i.e., the substituents are oriented alternately and regularly above and below the plane containing the polymer chain. Syndiotacticity can also be determined through the use of NMR. In NMR nomenclature, a syndiotactic pentad is represented by . . . rrrr . . . in which each "r" represents a "racemic" dyad, i.e., successive substituents on alternate sides of the plane. The percentage of "r" dyads in the chain determines the degree of syndiotacticity of the polymer.
There are other variations in polymer structures as well. One such variant is the so-called hemi-isotactic polymers. Hemi-isotactic polymers are ones in which every other pseudo-asymmetric carbon atom has its substituent oriented on the same side relative to the plane containing the polymer backbone. While, the other pseudo-asymmetric carbon atoms can have their substituents oriented randomly either above or below the plane. Since only every other pseudo-asymmetric carbon is in an isotactic configuration, the term hemi is applied.
Isotactic and syndiotactic polymers are crystalline polymers and are insoluble in cold xylene. Crystallinity distinguishes both syndiotactic and isotactic polymers from hemi-isotactic or atactic polymers that are soluble in cold xylene and are non-crystalline. While it is possible for a catalyst to produce all four types of polymers (atactic, hemi-isotactic, isotactic and syndiotactic), it is desirable for a catalyst to produce predominantly or essentially exclusively isotactic or syndiotactic polymer with very little atactic polymer and few stereochemical defects.
In recent years, numerous patents and applications have been filed relating to the use of metallocene and constrained geometry catalyst for the efficient production of polyolefins and for the efficient formation of tactioselective polyolefins including iso, hemiso and syndio tactic polypropylene.
Constrained geometry catalysts where one the cyclopentadienyl groups has been replaced by a hetero atom ligand such as an amino or phosphino anion are described in the following U.S. Pat. Nos.: 5,453,410, 5,399,635, and 5,350,723, incorporated herein by reference.
Several catalysts that produce isotactic polyolefins are disclosed in U.S. Pat. Nos. 4,794,096 and 4,975,403, as well as European Pat. Appl. 0,537,130, incorporated herein by reference. Several catalysts that produce syndiotactic polyolefins are disclosed in U.S. Pat. Nos. 3,258,455, 3,305,538, 3,364,190, 4,852,851, 5,155,080, 5,225,500, and 5,459,117, incorporated herein by reference.
Besides neutral metallocenes, cationic metallocenes are known to result in polymers with varying degrees of tactiospecificity. Cationic metallocene catalysts are disclosed in European Patent Applications 277,003 and 277,004, incorporated herein by reference. Catalysts that produce hemi-isotactic polyolefins are disclosed in U.S. Pat. Nos. 5,036,034, incorporated herein by reference.
In addition to polymers of mono-olefins homopolymers, polymerization catalysts for preparing copolymers of mono-olefins or polymers of di-functional olefins or copolymers of di-functional olefins and mono-olefins can be prepared using coordinated metal catalysts including metallocene catalysts.
Although these catalysts are efficient for the production of polyolefins, most of these catalysts are designed to operate a fairly modest temperature (about 70.degree. C.) in the gas phase and are not suitable for high temperature solutions polymerization where the temperature can exceed about 120.degree. C. In such condensed phase polymerization, a process using a set of haffocene catalysts that can produce tactioselective to tactiospecific catalyst in condensed phase polymerizations would represent an advancement in the art.